1. Field of the Invention
The present invention relates to Fourier transform infrared (FTIR) spectrometers and more particularly to a method and apparatus for improving the sensitivity of an FTIR spectrometer by performing real-time spectral shifts on measured data based on the centerline position of known elements in the environment.
2. Description of the Prior Art
FTIR spectrometers are well known in the prior art. A typical FTIR spectrometer based on a Michelson interferometer is illustrated in FIG. 1. Typically, such FTIR systems have been used in a laboratory setting under controlled conditions to make qualitative measurements based on spectral signature analysis. However, within the last two decades, the FTIR spectrometer has been used to perform quantitative analysis of elements in an open and uncontrolled atmosphere. Such "open-path" applications include industrial monitoring of pollutants from a smoke stack and military monitoring of chemicals used in war zones. However, once the controlled environment of the laboratory is left behind, variables within the measurement path must be neutralized in order to achieve accurate results.
In the FTIR spectrometer of FIG. 1, an infrared source 2 reflecting or emitting from a sample to be analyzed is directed onto a semitransparent optical beam splitter 4. The beam splitter 4 reflects approximately half (some loss due to losses in the beam splitter) of the infrared signal to a moving mirror 6 and transmits the remaining half of the signal to a fixed mirror 8. The moving mirror 6 is orthogonally aligned to the fixed mirror 8 and the beam splitter 4 is interposed between the mirrors at a 45.degree. angle. The signals reflected off the fixed mirror 8 and the moving mirror 6 are combined by the beam splitter 4 and are reflected onto a detector 10. As the moving mirror 6 travels in a reciprocating fashion on a line parallel to the fixed mirror 8, the pathlength of the signals reflected by the moving mirror 6 varies. This creates a shift in the relative phase angles of the signals being combined by the beam splitter 4. This combination results in both constructive and destructive interference at the detector 10. This interference creates a position versus magnitude signal known as an interferogram. The detector 10 translates the optical interferogram into an analog voltage which is received by an analog to digital (A/D) converter 12. The A/D converter 12 creates a digital signal representing the detected optical interferogram signal. The digital signal from the AID converter 12 is coupled to a computer 14 for digital signal processing to determine the concentration level of chemical species in the atmosphere. A helium-tieon (HE-NE) laser 16 is used as a signal source for a secondary interferometer 18 to generate a single frequency sinusoidal time reference. The time reference from the HE-NE laser 16 is received by the A/D converter 12 and functions as a synchronizing clock for the A/D converter 12.
The operation of a traditional FTIR spectrometer is illustrated in the block diagram flow chart of FIG. 2. This figure begins with an illustration of the previously described interferogram 20. The computer 14 is used to perform a fast Fourier transform (FFT) 22 which translates the time domain interferogram of block 20 into a frequency domain, single-beam spectrum 24. From the single beam spectrum 24, both a background spectrum (baseline spectrum) 26 and analytical spectrum 28 are derived. From the background and analytical spectra, a transmission spectrum 30 is calculated by dividing the analytical spectrum by the background spectrum. Finally, an absorption spectrum 32 is calculated as the negative logarithm of the transmission spectrum.
The background spectrum 26 is required to reduce baseline variations which can contribute to errors in open-path, centerline measurements. The background spectrum 26 is used to convert the subsequent analytical spectra 28 into compensated absorption spectra 32. This eliminates spectral distortions which may result from the characteristics of the source 2, beam splitter 4, detector 10, and interfering components within the measurement atmosphere. Ideally, the background spectrum 26 would be acquired by sampling the target atmosphere at a time when the target gas to be measured is not present. However, in an open-path system, this is not always possible and indirect background spectrum generation techniques are required. One such technique is known as synthetic background spectrum generation. In this method, a background spectrum 26 is created by taking samples of the original spectrum at points where no components are expected, then generating a curve to fit these sample points. A suitable curve fitting function is the polynomial defined by EQU y=ax.sup.2 +bx+c
where a, b, and c are coefficients to be calculated based on a least squares curve fitting algorithm.
As previously mentioned, open-path measurements are subjected to variables not encountered in a laboratory setting. One such variable in open-path FTIR measurement is referred to as "wave number shift." A spectrometer works on the principle that a given chemical species has a spectral signature having components at predetermined wavelengths. To identify and quantify specific elements, the output of the spectrometer is evaluated at those characteristic wavelengths. A typical evaluation method requires direct subtraction of the measured samples against a known reference signal. If, because of environmental and/or measuremental variables, the wavelength of the components of the measured sample does not match that of the reference spectrum, errors in analysis will occur. FIG. 3 illustrates the typical output that results when direct subtraction is performed between a wave number shifted measured signal and a non-shifted reference signal. As FIG. 3 clearly shows, a wave number shift results in a non-zero differential error when the shifted and non-shifted signals are subtracted. Previously, operators of a spectrometer would manually compensate for these wave number shifts by performing off-line compensation of measured signals.